As the damage caused by climate change increases all over the world, air pollution problem has recently attracted considerable attention. Among materials related to global warming, nitrous oxide (N2O) contributes to the greenhouse effect, 310 times as much as carbon dioxide (CO2), is a compound stable in air, and is presumed to stay in the air for 20˜100 years. Further, nitrous oxide (N2O) destroys the ozone layer of the stratosphere. Therefore, it is known that, when the amount of nitrous oxide (N2O) increases two fold, the amount of ozone decreases by 2%.
Nitrogen oxides are necessary by-products of the procedure of burning fossil fuels or incinerating various wastes, and generate photochemical smog and various secondary pollutants (O3, PAN, etc.) through a photochemical reaction that takes place in the air. That is, nitrogen oxides are very important in terms of air pollution. In order to cope with such crises caused by climate change, it is required to effectively remove nitrous oxide (N2O) as a greenhouse gas together with nitrogen oxides (NOX).
Referring to conventional technologies related to the decomposition of N2O and NOX, there are proposed a method of controlling the temperature of a combustion furnace, a method of recirculating exhaust gas, a method of mounting a special burner, and the like. However, these methods are poor in terms of performance. Thereafter, selective catalytic reduction (SCR) has been put to practical use, but has the problems of high equipment cost and its performance worsening with aging of the catalyst.
Patent document 1, filed by Alini et al, discloses a method of decomposing N2O gas at a temperature of 400° C. or more using a rhodium (Rh)-containing hydrotalcite catalyst. However, this method is problematic in that N2O at a very low concentration of 200 ppm or less must react at a high temperature of 400° C. or more and the rhodium (Rh)-containing hydrotalcite catalyst is expensive.
Patent document 2 discloses a method of catalytically reducing nitrogen oxides included in exhaust gas containing light hydrocarbons using the exhaust gas as a reductant and using β-Zeolite exchanged with a cobalt salt as a catalyst. However, this method is problematic in that the conversion ratio of N2O is low although N2O is reacted at high temperature using a large amount of a reductant which is two or more times larger than that used with NOX.
According to the paper [Reduction of N2O with CO Over FeMFI zeolites., Catal. 223(2004)] written by Javier P. R. et al., when N2O was decomposed using a zeolite catalyst coated with iron (Fe) in the presence of carbon monoxide (CO) as a reductant, the decomposition rate of N2O was improved compared to those of conventional technologies. However, there is a problem in that N2O must be decomposed at a high temperature of 400° C. or more.
Patent document 3 discloses a method of directly reducing nitrogen oxide by catalytic reduction using a catalyst in which a solid powdered carrier is supported with a metal. However, this method is problematic in that the conversion ratio of N2O is 50% or less even at a high temperature of 400° C. or more.
Patent document 4, filed by Schwefer et al., discloses a method of decomposing N2O and NOX at a temperature of 450° C. and a pressure of 6.5 bar using an iron-containing ZSM-5 catalyst and a reductant (ammonia). However, this method is problematic in that the conversion ratio of N2O is 64% and the conversion ratio of NOX is 78%, each of which is somewhat low, and in that the decomposition temperature and pressure are high.
In Patent document 5 filed by Nagata, Hihara et al., β-Zeolite exchanged with iron ion was impregnated with ferric nitrate and then calcinated at 500° C. for 5 hours to prepare an ion-exchanged catalyst containing about 4% of iron oxide, and then this catalyst was mixed with a binder and then applied onto a honeycomb support, and then the performance of the catalyst was tested. In this case, when NOX included in the exhaust gas of a diesel engine was decomposed at the low temperature of 200° C. using urea as a reductant, there is a problem in that the conversion ratio of NOX is about 43%, which is low.
Patent document 1: European Patent Publication No. 1262224
Patent document 2: Korea Patent No. 0359675
Patent document 3: Korea Patent No. 0408880
Patent document 4: U.S. Pat. No. 7,462,340
Patent document 5: U.S. Pat. No. 7,501,105